1. Field of the Invention
The present invention relates to an electrostatic latent image evaluation device, an electrostatic latent image evaluation method, an electrophotographic photoreceptor and an image forming device, in particular, to an electrostatic latent image evaluation device and an electrostatic latent image evaluation method, which measure an electric charge distribution or an electric potential distribution on a surface of a dielectric body with high accuracy in micron order.
2. Description of the Related Art
It is well known that electric charge on a surface of a dielectric body is spatially scattered in the dielectric body. For this reason, on the surface of the dielectric body, the electric charge is distributed in the surface direction along the surface. The electric charge includes not only electrons but also ions. In addition, a dielectric body to which a voltage is applied by a conductive section provided in the surface of the dielectric body and in which an electric potential distribution is generated on or near the surface of the dielectric body is used as the dielectric body.
There is a conventional method of measuring the surface potential of a dielectric body. In such a method, a sensor head is moved closer to the dielectric body having an electric potential distribution, an electrostatic attractive force and an induction current generated at that time are measured, and the measured value is converted into an electric potential distribution. This method, in principle, has a low distance resolution which is about several millimeters, and can not obtain resolution of 1 μm order.
A method of measuring an electric potential by using an electron beam with a distance resolution of 1 micron order is known as a method of evaluating an LSI chip. This evaluation method can be used for an LSI in which the input and output electric potential is limited to a low electric potential of about +5V, but this evaluation method can not correspond to an object in which electric potential on a dielectric surface of an electrophotographic photoreceptor reaches several hundred volts to several thousand volts.
On the other hand, for example, an invention described in JP H03-049143A is known as a method of visualizing an electrostatic latent image (obtaining an image) by an electron beam. In this method, a sample is partially scanned by an electron beam in advance, a back bias value which can obtain an appropriate electric potential contrast is detected by the number of obtained secondary electron signals, the power source is controlled so as to apply this back bias to the sample, the sample is scanned again by an electron beam, and image signals showing an electrostatic latent image are output by the obtained secondary electron signals.
In the invention described in JP H03-049143A, the sample is limited to an LSI chip or a sample which can memorize and hold an electrostatic latent image. For this reason, this invention can not correspond to a general electrophotographic photoreceptor generating dark decay of an electrostatic latent image, which is a target of the present invention, because of the following reasons. More particularly, since a general dielectric body semi-permanently holds electric charge, when obtaining an image of an electrostatic latent image after forming electric charge distribution (electrostatic latent image), the obtained image is not temporally changed. However, in the electrophotographic photoreceptor, since a resistance value is not infinity, the electric charge can not be maintained for a long period of time. For this reason, the dark decay of the electrostatic latent image occurs, and the surface potential is decreased with time. The electrophotographic photoreceptor can generally maintain electric charge for about 10 to 60 seconds even in a dark room. Therefore, after charging and exposing an electrophotographic photoreceptor, the electrostatic latent image disappears in a preparation step of obtaining an image of an electrophotographic photoreceptor by means of an electron microscope (SEM). Due to this feature, in the electrophotographic photoreceptor, it is necessary to obtain an image within 3 seconds at the most after forming the electrostatic latent image.
An x-ray microscope is also described in JP H03-200100A. In the x-ray microscope, an x-ray source is disposed to face one surface of an electrophotographic photoreceptor which photographs an x-ray transmission image of a sample, a scanning electron beam emitter is disposed to face the other surface of the electrophotographic photoreceptor, a secondary electron detector which detects a secondary electron emitted from a photoconductive layer of the electrophotographic photoreceptor is disposed, and a controller which converts the output of the secondary electron detector into an image is disposed.
However, the invention described in JP H03-200100A is completely different from the electrostatic latent image evaluation device and the electrostatic latent image evaluation method of the present invention in a wavelength of a beam. In the invention described in JP H03-200100A, it is impossible to form an electrostatic latent image having an arbitrary line pattern, a desired beam diameter and a desired beam profile, and an object of the present invention can not be achieved.
Consequently, the present inventors invented a device or a method of obtaining an electrostatic latent image even in a photoreceptor having dark decay, and filed patent applications. The inventions described in JP2003-295696A, JP2004-251800A, JP2008-233376A are examples.
In the meanwhile, in an image forming device such as a digital PPC (Plain Paper Copier) and a laser printer, the quality in each process such as charging, exposing, developing, transferring and fusing significantly affects the quality of a final output image. For this reason, in order to obtain a high quality image, it is necessary to improve the quality in each process. It is extremely important to evaluate the quality of an electrostatic latent image after exposing in order to obtain a high quality image. However, it is difficult to evaluate the ability of forming an electrostatic latent image because a mechanism to convert exposure energy into an electrostatic latent image is unknown. Therefore, the most appropriate value is not set for a specification of each section, and the specification of each section becomes an excessive specification in order to obtain a predetermined image quality, resulting in the increase in the costs.
The invention according to each of the above patent applications by the present inventors relates to a system capable of measuring an electrostatic latent image in a μm scale by reproducing in a vacuum device an electrostatic latent image formed on a photoreceptor for use in an electrophotographic process. By inventing and developing such a system, an environment capable of evaluating the ability of forming an electrostatic latent image, which was impossible in the conventional art, can be established.